Signed-off-by: Thomas Haller <thaller@redhat.com>

Move the ckecks for nmc_arg_is_help to the beginning of the
checks for command matches.

Up to now, no command begins with 'h', so this has no behavioral
change whatsoever. But imagine a command that begins with 'h'
(for example `nmcli general hostname`), in that case `nmcli general h`
should still show the help, as users might be accustomed to this
abbreviation.

Signed-off-by: Thomas Haller <thaller@redhat.com>

Ensure in matches() that a non empty cmd is given, otherwise
currently

  nmcli general -

matches to '-h' and is thus treated as

  nmcli general -h

Signed-off-by: Thomas Haller <thaller@redhat.com>

** (process:25157): CRITICAL **: nmc_activate_connection: assertion `NM_IS_CONNECTION (connection) || ifname' failed
Error: unknown error.

Regression caused by 42c7ea85a17a7e7459942f356399e36308011c86.

Passes a NULL connection to nm_client_activate_connection() allowing
NetworkManager to pick the best available connection for the interface.

This command allows activating the edited connection.

Monitoring the progress of the activation is a bit complicated by the fact
that the callback activate_connection_editor_cb() is invoked in main loop
thread, and not in the editor thread itself.

https://bugzilla.redhat.com/show_bug.cgi?id=1004883

Found with Coverity.

Only complete the setting name if it is at the very first
position after the connection.

e.g. complete the settings name in the case
  $ nmcli connection modify em1 connec<TAB>
but not at
  $ nmcli connection modify em1 connection.autoconnect <TAB>

Signed-off-by: Thomas Haller <thaller@redhat.com>